Aramid insulating papers are also referred to as aramid papers, and generally classified into meta-aramid insulating papers and para-aramid insulating papers. Among others, the meta-aramid insulating papers exhibit good and persistent thermal stability, remarkable flame retardance, outstanding high temperature resistance and electrical insulation properties, and excellent chemical stability and mechanical properties, and therefore find a wide range of applications in aerospace, transportation and electronic industries as structural materials, insulating materials and filter materials.
The aramid insulating papers are generally made by blending and forming aramid short fibers and aramid pulp in a given ratio into papers, and pressing, drying and high temperature calendering the papers. The short fibers provide mechanical properties to the papers, and the aramid pulp comprises pulp fibers with rich capillary fibers to function as a binder. After the formation of the paper, heating and pressing the paper allow the aramid pulp having a lower melting point to melt, thereby binding to the short fibers.
Presently, a large portion of the aramid insulating papers are made of para-aramid short fibers, meta-aramid pulp and a third fiber or a binder. For example, Chinese patent no. ZL93106746.4 discloses a paper which is synthesized by para-aramid short fibers and meta-aramid pulp; Chinese patent no. ZL99125156.3 discloses papers made by aromatic polyamide fibers and a binder; Chinese patent no. ZL200410026569.1 discloses papers which were made using aromatic polyamide fibers, poly(p-phenylene benzobisoxazole) fibers and fibrillating fibers as raw materials. The above patents emphasize the proportioning of the aramid fibers and the heat-pressing technology, and the final papers in these patents are mixed papers of different materials.
Another Chinese patent application no. 200610043659.4 provides a method for preparing meta-aramid papers using meta-aramid fibers only, comprising the operations of adding 5-95 wt % meta-aramid short fibers into water to form a short fiber slurry and adding 5-95 wt % meta-aramid fibrids into water to form a fibrid slurry; blending the two slurries; forming the blended slurries into papers on a papermaking machine; pressing, drying and high temperature calendering the papers to make final papers. This patent application has solved the problem of dispersing the meta-aramid fibers in the raw material stage, but fails to elaborate the technology of papermaking.
Ultrasonic waves are sonic waves having frequency higher than 20000 Hz. They exhibit good orientation and high penetrability, are able to radiate centralized acoustic energy and have a long propagation length in water. Therefore, the ultrasonic waves are widely used in military affairs, medicine, industry, and agriculture. It is well known that ultrasonic waves are characterized by directional straight propagation, cavitation and mechanical effects, and contribution to dispersion of solids, due to high frequency and short wave length.
The dispersion and flocculation of the paper pulp are known to seriously affect the mechanical strength and uniformity of the aramid insulating papers, and the degree of orientation of the short fibers also has a great impact on the mechanical strength of the aramid insulating papers. The invention has solved the above problems by utilizing the ultrasonic waves in the papermaking process to produce the aramid papers exhibiting enhanced mechanical strength and increased smoothness and uniformity.